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Copper Production

Modern Copper Production

80% of the entire copper are produced using pyrometallurgical methods based on complete fusing of the whole mass. During this process copper is concentrated in sulfides (matte) and gang mineral goes to the slag, because of the coppers higher sulphur affinity. Matte, basically Cu2S, FeS, is separated from slag by setting.

Compressed air is blown through the layer of liquid copper matte placed into the converter. On the first step iron sulphide is oxidized. Quartz is added for binding the iron oxides and removing them in form of converter slag. After that copper sulphide is oxidized yielding metal copper on SO2. This crude copper is cast. The billets are then undergoes to fire refining based on oxophilicity of impurities stronger than the copper's; in this way Iron, Zinc, Cobalt and, partially, Nickel are removed with slag, and Sulphur - with gases such as SO2. The second major problem after getting slag out is to get rid of excess Cu2O. It is solved by deoxidizing copper to the correct percentage of Cu2O. This is done by the process called poling. After the copper was molten and its slag skimmed off, pieces of wood, usually birch or deal-board, thrust into the molten metal after which the metal is poured in flat casts. Then the stage of electrolytic refining begins during which copper anodes are cast using smeltered copper in the tank with solution of CuSO4 acidated by H2SO4.

The other classical methods employed in the manufacture of copper from its ores may be classified in three main divisions -
  1. dry methods, for ores with more than 4 per cent, of copper;
  2. wet methods, for ores with less than 4 per cent, of copper;
  3. electrometallurgical methods.

Dry Methods of Copper Production

The ore is first calcined or roasted, a part of the arsenic and antimony present being eliminated as oxide or sulphide, and part remaining as arsenate or antimonate. Simultaneously, a considerable proportion of the sulphur is removed, the roasted ore consisting chiefly of sulphides, sulphates, and oxides. It is then smelted to a mixture of copper and iron sulphides, known as copper-matte or coarse-metal, the process being carried out either in a blast furnace (German or Swedish process) or in a reverberatory furnace (English process). The matte is then smelted with coke and siliceous fluxes to slag off the iron, the operation being performed in a blast furnace (German process), a reverberatory furnace (English process), or a converter (Bessemer process). The product is an impure copper sulphide, called blue-metal when it contains iron, pimple- metal when free copper and copper oxide are present, and fine-metal or white-metal when it consists of a fairly pure copper sulphide with about 75 per cent, of the metal. This product is again smelted to form coarse-copper, containing about 95 per cent, of the metal. The coarse- copper is then refined. In the American or pyritic smelting process the raw ore is smelted directly to matte in a blast furnace. The various processes are named after the countries in which they have been most developed, but no individual process is limited to the country of its origin.

The German or Swedish process involves five steps -
  1. roasting the ore;
  2. smelting the roasted ore to matte in a blast furnace;
  3. roasting the matte;
  4. smelting the matte in a blast furnace with coke and fluxes to black-metal or coarse-metal;
  5. refining the coarse-metal.
During the process the siliceous material and part of the iron are converted into a fused silicate-slag, floating on a heavier layer of copper matte consisting of a fused mixture of cuprous sulphide and ferrous sulphide. The reduction to copper is effected chiefly by carbon monoxide or carbon, and not by the sulphur present. To concentrate the matte, the cycle of oxidation by roasting and reduction by smelting is repeated several times, the iron being gradually removed as a fused slag of ferrous silicate.

The English process comprises six operations -
  1. calcination;
  2. smelting to matte in a reverberatory furnace;
  3. roasting the matte;
  4. smelting the matte in a reverberatory furnace to white-metal or fine-metal;
  5. conversion of the fine-metal into coarse-copper or blister-copper, sometimes after a preliminary calcination;
  6. refining the coarse-copper.
In this process the calcined ore is smelted in a reverberatory furnace, the reduction being effected by the interaction of the sulphides and oxides or sulphates, with evolution of sulphur dioxide, thus yielding a more concentrated matte than the blast-furnace process -

Cu2S+2Cu2O = 6Cu+SO2;
Cu2S+2CuO = 4Cu+SO2;
Cu2S+CuSO4 = 3Cu+2SO2.

The siliceous lining of the hearth converts the ferrous oxide into a slag of ferrous silicate. By altering the air-supply to the furnace, the matte is subjected to alternate oxidation and reduction. Both the German process and the English process depend on the fact that copper has a greater affinity for sulphur than for oxygen, and iron a greater affinity for oxygen than for sulphur.

The Welsh process differs from the English method in the enrichment of the matte by smelting with copper slags formed in subsequent operations.

The Anglo-German process is a combination of the English and German methods. After calcination the ore is smelted in a shaft furnace, and the matte is concentrated in a reverberatory furnace. The subsequent smelting to coarse-metal can be effected in either type of furnace.

The concentration of the matte and its subsequent smelting to coarse-copper are also effected by the Bessemer process, a modified type of Bessemer converter with the side tuyeres raised about 10 inches above the bottom lining being employed. The process finds extensive application, a large proportion of the arsenic and antimony being eliminated. A serious item of expense is the renewal of the siliceous lining of the converter, the silica required for slagging the iron being provided from this source.

In localities where fuel is expensive, pyritic smelting has been considerably developed. It was devised by John Hoi way in 1878.

The ore is roasted in an oxidizing atmosphere in the upper part of the blast furnace, two types of procedure being in vogue. The heat can be maintained by the combustion of the sulphur in the ore, no external fuel or hot blast being employed; or the process is facilitated by addition of fuel, or by a hot blast, or by both modifications simultaneously.

Ores not containing sulphur, such as the oxide and carbonate, are reduced with coke in a blast furnace. The impurities associated with native copper are removed by slagging in a reverberatory furnace.

The reduction of copper ores in the electric furnace has been studied by Juschkewitsch. Stephan has given an account of its application to the extraction of copper from its ores.

The unchallenged supremacy of the United States of America in copper production is attributed by Carpenter to two causes -

"A. Improvements effected in that country since 1880 in the technique of copper smelting and refining, enabling ores of very low grade to be worked economically.
B. The enormous reserves of ore existing in Arizona, Montana, Michigan, Utah, Tennessee, Mexico, and Chile.

The developments in manufacture are summarized by Carpenter as follows -
  1. Great improvements in mining and ore-dressing operations (mechanical concentration).
  2. The use of mechanically rabbled roaster furnaces.
  3. The manufacture of sulphuric acid both from blast-furnace and roaster gases.
  4. The blast-roasting and sintering of sulphide fines.
  5. The practical application of the pyritic smelting principle to suitable raw ores, and to practically all copper mattes.
  6. An enormous increase in the capacity and output both of blast and reverberatory furnaces.
  7. The recovery of metal values from the furnace waste-gases.
  8. The adoption of electrolytic refining, followed in many cases by furnace refining with the recovery of precious-metal values in the copper which more than suffice to pay for their extraction.
  9. The application of either leaching or flotation processes or both to low-grade ores, tailings, etc."

The chief impurities present in coarse-copper are iron, lead, zinc, nickel, cobalt, bismuth, arsenic, antimony, sulphur, selenium, and tellurium. The dry refining process involves an oxidizing fusion in a reverberatory furnace, the impurities being either volatilized or slagged. Part of the cuprous oxide formed by oxidation dissolves in the metal, rendering it hard and brittle. This oxide is partly reduced by interaction with the cuprous sulphide still present. The molten mass is poled with a pole of green wood, the evolved gases facilitating the elimination of the sulphur dioxide, and partially reducing the cuprous oxide to copper. To finish the reduction, the copper is covered with a layer of wood-charcoal or anthracite, and again poled. Excessive poling is deleterious, causing the reduction of arsenates and antimonates to the corresponding elements, and thus deteriorating the qualityof the copper. The soft, malleable metal is known as tough-pitch copper, and displays a lustrous, silky fracture. Several varieties of copper are recognized in the trade - bean-shot copper is formed by pouring the metal into hot water; feathered-shot copper by pouring into cold water; rosette copper by cooling the surface of the fused metal with water, and removing the thin, dark red crust formed; japan copper by casting into ingots, and cooling rapidly with water, its colour being purple-red; tile copper, an impure form produced by refining the first tappings; best- selected copper, a purer type.

Wet Methods of Copper Production

Wet methods of extraction are applied to low-grade ores containing not less than 0.25 to 1 per cent, of copper, or to products containing copper associated with gold and silver. For ores containing copper as oxide or carbonate, the solvents employed are sulphuric acid, hydrochloric acid, and ferrous chloride.

The original Hunt-Douglas process involves treatment of the ore at 70° C. with a solution of ferrous chloride produced by the interaction of sodium chloride and ferrous sulphate. The ferrous chloride converts copper oxide and carbonate into a solution of cuprous and cupric chlorides, ferric hydroxide being precipitated and carbon dioxide evolved. After filtration to remove ferric hydroxide the copper is precipitated by addition of scrap iron, the ferrous chloride thus regenerated being available for further use in the first stage of the process. The amount of iron required is small, but the method is handicapped by difficulty in filtering the ferric hydroxide, by the tendency of the copper solution to undergo atmospheric oxidation, and by the fact that any silver present is dissolved.

In the modern Hunt-Douglas process the ore is leached with dilute sulphuric acid, and the copper converted into cupric chloride by addition of ferrous chloride or calcium chloride. The use of the calcium salt entails removal of the calcium sulphate by filtration. The cupric salt is precipitated as cuprous chloride by reduction with sulphur dioxide, and the precipitate is converted into metallic copper by treatment with iron, or into cuprous oxide by the action of milk of lime. In this process the amount of iron needed is proportionately small, ferric hydroxide is not precipitated, and silver is not dissolved.

When the ore contains cuprous sulphide this salt is converted into a soluble form cupric sulphate, soluble in water; cupric oxide, soluble in hydrochloric or sulphuric acid; cupric chloride, soluble in water; or cuprous chloride, soluble in solutions of metallic chlorides.

Conversion into the sulphate is effected by weathering, a slow and expensive process; by calcination, for ores containing a high percentage of iron pyrites; by calcination with ferrous or aluminium sulphate; or by calcination with ferric sulphate as an adjunct to the weathering process.

The transformation into oxide is carried out by calcination in a reverberatory furnace, or, if the sulphur is to be recovered, in a muffle furnace. Its economical working is contingent on the availability of a cheap supply of hydrochloric or sulphuric acid.

The formation of the chlorides is effected in the dry way by calcination with sodium chloride; or in the wet way by interaction with ferrous chloride and hydrochloric acid or with ferric chloride. The wet way is only adopted if fuel is scarce, or the escape of noxious vapours into the atmosphere is not permissible. In the dry method the ore is oxidized by a preliminary roasting, and then chloridized by calcination with sodium chloride or Abraum salts in a furnace of the reverberatory or muffle type, the principal product being cupric chloride. The Dotsch modification of the wet process, worked at Rio Tinto, depends on the action of ferric-chloride solution on a mixture of the ore with sodium sulphate and ferric chloride. The liquid drawn off from the bottom of the heaps of ore contains cuprous chloride in solution as a complex salt. The copper is liberated by the action of iron, the ferrous chloride simultaneously formed being chlorinated in towers to ferric chloride, and the product employed for moistening the heaps of ore.

After lixiviation of the sulphate, oxide, or chlorides obtained by these methods, the copper is precipitated by the process already described. If the cement copper thus obtained contains over 55 per cent, of the metal, it is refined directly; if the percentage is lower, it is first smelted with matte or calcined copper pyrites.

Electrometallurgical Methods of Copper Production

Several processes for the extraction of copper by electrometallurgical methods have been devised. The Marchese patent aimed at the decomposition of copper matte and deposition of the copper simultaneously in a sulphuric-acid electrolyte with a copper-matte anode and a copper cathode. In Siemens and Halske's patent the copper of the ore was oxidized to the cupric state by an acid solution of ferric sulphate, and the copper deposited electrolytically in a second vessel with a diaphragm separating the cathode and the anode. Neither - process has been commercially successful.

Hopfner's patent is worked on a small scale. The ore is extracted with a solution of cupric chloride and calcium or sodium chloride, the copper of the cupric salt and that of the cuprous sulphide being converted into cuprous chloride, with precipitation of sulphur. The copper is deposited from the cuprous-chloride solution in a cell fitted with a diaphragm, a copper cathode, and a carbon anode, the extracting liquid being regenerated at the anode, and being thus available for further extraction of the ore.

Processes devised by Carmichael and by Browne have been employed in Canada for the extraction of copper.

The process patented by Jumau involves the extraction of the roasted ore with an ammoniacal solution of ammonium sulphate or sulphite. Sulphurous acid reacts with the solution thus formed, precipitating either cuprous sulphite or cupro-cupric sulphite

3Cu(OH)2+3SO2 = CuSO3,Cu2SO3+H2SO4+2H2O.

The precipitated sulphite is redissolved in an ammoniacal solution of ammonium sulphate or sulphite, and the solution electrolyzed.

Electrolytic Refining

In 1865 Elkington patented a process for the electrolytic refining of copper, similar in principle to the method employed at the present day. In the modern process the bath is a solution containing 12 to 20 per cent, of copper sulphate and 4 to 10 per cent, of sulphuric acid. A fairly pure anode of copper containing small amounts of silver, gold, arsenic, antimony, iron, and other impurities is employed, the metal being deposited on a copper cathode. A pure and coherent deposit of copper is obtained with a low current-density of 0.0043 to 0.0484 ampere per sq. cm., the noble metals being deposited in the anode mud, and the other impurities remaining partly in this mud, and partly entering into solution. If the current-density be too low, the deposited copper is pale and brittle; if too high, it is dark-brown and spongy. Constant attention must be paid to the 'composition and degree of acidity of the electrolyte, both important factors influencing the nature of the deposit. With rotating cathodes a good deposit is obtained with currents of high density, but in practice this modification is precluded by the disturbance of the anode mud, the solid particles in the electrolyte causing the formation of nodular growths on the deposited copper. The yield of copper obtained by the electrolytic method usually corresponds with a current-efficiency of 94 to 96 per cent., although it is possible to attain an efficiency of 98 per cent. The bath is usually maintained at a temperature of 40° to 50° C.

Jumau's process (ut supra) for the electrolytic extraction of copper from its ores is also applicable to the production of pure copper from solutions of its compounds. The cupric sulphite or cupro-cupric sulphite precipitated from the copper solution by the action of sulphurous acid or a sulphide is decomposed by sulphuric acid into cupric sulphate and metallic copper. The metal thus liberated is pressed into a form suitable for an anode, and refined electrolytically.

Copper Refining by other Methods

Various other methods are available for the purification of copper. An example is the ready reduction of cuprous chloride by soft iron, a substance without action on cupric chloride. Aluminium slowly reduces a warm solution of cupric sulphate. Vigoroux recommends a method depending on the action of aluminium on a solution of copper in concentrated hydrochloric acid.

Copper-plating is effected by the electrolytic deposition of the metal from a bath of the cyanides of copper and potassium, and is an important industry.

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